late 15c., duelle (from late 13c. in Latin form), "a single combat," also "a judicial single combat," from Medieval Latin duellum "combat between two persons," from Latin duellum "war," an Old Latin form of bellum (see bellicose). The Old Latin word was retained in poetic and archaic language and apparently given a special meaning in Medieval or Late Latin of "one-on-one combat" on fancied connection with duo "two."
Sometimes also in Italian form duello. The English word by 1610s had taken on the specialized sense of "premeditated and pre-arranged single combat involving deadly weapons in the presence of at least two witnesses." General sense of "any contest between two parties" is from 1590s.